Problem: On the first 4 tests of his chemistry class, Michael got an average score of 83. What does he need on the next test to have an overall average of 85?
Let his score on the next test be $x$ The sum of all of his scores is then $4 \cdot 83 + x$ The same sum must also be equal to $5 \cdot 85$ Solve: $x = 5 \cdot 85 - 4 \cdot 83 = 93$.